Effects of cholestyramine feeding on tissue lipase activities and plasma fatty acids in the pregnant rat

Rats fed a non-absorbable bile acid binding resin (cholestyramine) throughout gestation had decreased activities of adipose tissue lipoprotein lipase (LPL), hepatic triacylglycerol lipase and a heparin-releasable placental lipase distinct from LPL, when assayed at near-term gestation. The fetal plasma and liver triacylglycerol concentrations were not altered. The fetal liver total lipid and plasma triacylglycerol, however, had reduced levels of n-6 and n-3 series fatty acids, suggesting decreased availability of maternal dietary-derived essential fatty acids. These studies suggest that cholestyramine feeding may alter triacylglycerol flux and the quantity or type of maternal fatty acids available for placental transfer. The resin has application for in vivo study of the effects of maternal lipid transfer on the regulation of fetal hepatic lipid synthesis.     

Lipolysis: pathway under construction

PURPOSE OF REVIEW: The lipolytic catabolism of stored fat in adipose tissue supplies tissues with fatty acids as metabolites and energy substrates during times of food deprivation. This review focuses on the function of recently discovered enzymes in adipose tissue lipolysis and fatty acid mobilization. RECENT FINDINGS: The characterization of hormone-sensitive lipase-deficient mice provided compelling evidence that hormone-sensitive lipase is not uniquely responsible for the hydrolysis of triacylglycerols and diacylglycerols of stored fat. Recently, three different laboratories independently discovered a novel enzyme that also acts in this capacity. We named the enzyme 'adipose triglyceride lipase' in accordance with its predominant expression in adipose tissue, its high substrate specificity for triacylglycerols, and its function in the lipolytic mobilization of fatty acids. Two other research groups showed that adipose triglyceride lipase (named desnutrin and Ca-independent phospholipase A2zeta, respectively) is regulated by the nutritional status and that it might exert acyl-transacylase activity in addition to its activity as triacylglycerol hydrolase. Adipose triglyceride lipase represents a novel type of 'patatin domain-containing' triacylglycerol hydrolase that is more closely related to plant lipases than to other known mammalian metabolic triacylglycerol hydrolases. SUMMARY: Although the regulation of adipose triglyceride lipase and its physiological function remain to be determined in mouse lines that lack or overexpress the enzyme, present data permit the conclusion that adipose triglyceride lipase is involved in the cellular mobilization of fatty acids, and they require a revision of the concept that hormone-sensitive lipase is the only enzyme involved in the lipolysis of adipose tissue triglycerides.     

Regulation of lipid flux between liver and adipose tissue during transient hepatic steatosis in carnitine-depleted rats

Rats with carnitine deficiency due to trimethylhydrazinium propionate (mildronate) administered at 80 mg/100 g body weight per day for 10 days developed liver steatosis only upon fasting. This study aimed to determine whether the transient steatosis resulted from triglyceride accumulation due to the amount of fatty acids preserved through impaired fatty acid oxidation and/or from up-regulation of lipid exchange between liver and adipose tissue. In liver, mildronate decreased the carnitine content by approximately 13-fold and, in fasted rats, lowered the palmitate oxidation rate by 50% in the perfused organ, increased 9-fold the triglyceride content, and doubled the hepatic very low density lipoprotein secretion rate. Concomitantly, triglyceridemia was 13-fold greater than in controls. Hepatic carnitine palmitoyltransferase I activity and palmitate oxidation capacities measured in vitro were increased after treatment. Gene expression of hepatic proteins involved in fatty acid oxidation, triglyceride formation, and lipid uptake were all increased and were associated with increased hepatic free fatty acid content in treated rats. In periepididymal adipose tissue, mildronate markedly increased lipoprotein lipase and hormone-sensitive lipase activities in fed and fasted rats, respectively. On refeeding, carnitine-depleted rats exhibited a rapid decrease in blood triglycerides and free fatty acids, then after approximately 2 h, a marked drop of liver triglycerides and a progressive decrease in liver free fatty acids. Data show that up-regulation of liver activities, peripheral lipolysis, and lipoprotein lipase activity were likely essential factors for excess fat deposit and release alternately occurring in liver and adipose tissue of carnitine-depleted rats during the fed/fasted transition.     

Hormones and triacylglycerol metabolism under normoxic and ischemic conditions

Summary Fatty acids, the preferred substrate in normoxic myocardium, are derived from either exogenous or endogenous triacylglycerols. The supply of exogenous fatty acids is dependent of the rate of lipolysis in adipose tissue and of the lipoprotein lipase activity at the coronary vascular endothelium. A large part of the liberated fatty acids is reesterified with glycerol-3-phosphate and converted to triacylglycerols. Endogenous lipolysis and lipogenesis are intracellular compartmentalized multienzyme processes of which individual hormone-sensitive steps have been demonstrated in adipose tissue. The triacylglycerol lipase is the rate-limiting enzyme of lipolysis and glycerol-3-phosphate acyltransferase and possibly phosphatidate phosphohydrolase are the rate-limiting enzymes of lipogenesis. The hormonal regulation of both processes in heart is still a matter of dispute. Triacylglycerol lipase activity in myocardial tissue has two intracellular sources: 1, the endoplasmic reticular and soluble neutral lipase, and 2. the lysosomal acid lipase. Studies in our laboratory have indicated that whereas lipolysis is enhanced during global ischemia and anoxia, overall lipolytic enzyme activities in heart homogenates were not altered. In addition we were unable to demonstrate alterations in tissue triacylglycerol content and glycerol-3-phosphate acyltransferase activity under these conditions. Lipolysis, is subject to feedback inhibition by product fatty acids. Therefore all processes leading to an increased removal of fatty acids from the catalytic site of the lipase will stimulate lipolysis. These studies will be reviewed. In addition, studies from our department have demonstrated the capacity of myocardial lysosomes to take up and degrade added triacylglycerol-particles in vitro. Such a process, stimulated by Ca²⁺ and stimulated by acidosis, offers another physiological target for hormone actions.     

Cyclic AMP metabolism and lipolysis in cultured human adipocyte precursors

Triacylglycerol breakdown (lipolysis) results from a series of reactions culminated by activation of "hormone-stimulated" triacylglycerol lipase, an enzyme unique to adipose tissue. We have studied various components of the lipolytic process in human omental adipocyte precursors differentiating in culture. The levels of cyclic AMP, the "second messenger" of lipolytic hormones, were about sixfold higher in fat cell precursors than those in abdominal skin fibroblasts. L-Isoproterenol resulted in significant elevation of cyclic AMP levels in both cell types. Preincubation of intact adipocyte precursors with insulin resulted in significant enhancement of "low Km" cyclic AMP phosphodiesterase activity; in contrast, this hormone had no effect on fibroblast phosphodiesterase activity, a distinctive biochemical difference despite the morphological similarities between the two cell types during the early stages of adipocyte precursor maturation. Incubation of adipocyte precursors with isoproterenol resulted in the release of fatty acids into the medium, findings indicative of "hormone-stimulated" lipase activity and, hence, the operation of the entire "lipolytic cascade"; isoproterenol-stimulated lipolysis was inhibited by insulin. Release of fatty acids from fibroblasts was not observed. Thus, "hormone-stimulated" lipolysis and insulin stimulation of cyclic AMP phosphodiesterase activity are expressed during early stages of human adipocyte precursor differentiation.     

Effects of somatostatin-25 on lipid mobilization from rainbow trout,Oncorhynchus mykiss,liver and adipose tissue incubated in vitro. Comparison with somatostatin-14

Summary The physiological effects of the pancreatic peptides somatostatin-14 and somatostatin-25 on lipid metabolism in rainbow trout were evaluated by in vitro culture of liver and adipose tissue. The culture medium was subsequently analyzed for glycerol and fatty acid content and triacylglycerol lipase activity was measured within the tissues. Both somatostatin-14 and somatostatin-25 stimulated hepatic fatty acid and glycerol release within 3 h after treatment. Liver triacylglycerol lipase activity was elevated following treatment with somatostatin-14 (76% above control) or somatostatin-25 (94% above control). Somatostatin-14 and somatostatin-25 also significantly stimulated the release of fatty acid and glycerol from adipose tissue. Triacylglycerol lipase activity in adipose tissue also was enhanced by both somatostatins. These results indicate that somatostatin-14 and somatostatin-25 directly stimulate the mobilization of triacylglycerol from liver and adipose tissue, suggesting that these peptides are important systemic modulators of lipid metabolism in fish.Abbreviations bw body weight - cAMP cyclic adenosine monophosphate - FA ratty acids - fw fresh weight - GLU glucagon - INS insulin - MS-222 tricaine-methane sulphonate - SS-14 somatostatin-14 - SS-25 somatostatin-25 - TG triacylglycerol     

Lipolytic effects on diacylglycerol accumulation in human adipose tissue in vitro

When fragments of rat or human adipose tissue, or isolated adipocytes, are incubated with [14C]glucose in vitro, [14C]diacylglycerol accumulates rapidly: it comprises 20-50% of newly synthesized (14C-labeled) acylglycerols, compared to less than 1% diacylglycerol accumulated in the bulk lipid store in vivo. The experiments reported in this study were performed to test the possibility that agents that influence the rate of lipolysis might differentially affect the accumulation of di- and triacylglycerol in human adipose tissue, and perhaps account for the discrepancy between the early labeling and the later accumulation of diacyglycerol. Fragments of gluteal subcutaneous adipose tissue obtained from obese men and women were incubated with isoproterenol, epinephrine plus yohimbine, adenosine deaminase, or dibutyryl 3',5'-cyclic adenosine monophosphate to stimulate lipolysis. Tissue fragments were also incubated with clonidine, adenosine, or insulin to inhibit lipolysis. No agent had any effect on the rate of accumulation of newly synthesized triacylglycerol. The effects of these agents on the rate of lipolysis were negatively correlated with their effects on accumulation of newly synthesized diacylglycerol. Newly synthesized diacylglycerol may be preferentially hydrolyzed by hormone sensitive lipase. This increased susceptibility to lipolytic stimulation, compared to newly synthesized triacylglycerol, may account for the minute accumulation of diacylglycerol in adipose tissue in vivo.     

Lipid synthesis in vivo by tissues of the maternal and foetal guinea pig.

The rate of lipid biosynthesis in vivo was determined in pregnant guinea pigs after maternal and foetal injections of 3H2O. Synthesis in the maternal tissues was low and in the foetal liver and adipose tissues relatively high. In the foetal liver it reached a peak at about two-thirds of gestation, whereas that in the foetal adipose tissue occurred later. These results were used to support the view that lipid synthesis in the foetal guinea-pig liver at two-thirds of gestation is largely from short-chain fatty acids, whereas in foetal adipose tissue glucose is probably the major substrate.     

A low-protein, high-carbohydrate diet increases the adipose lipid content without increasing the glycerol-3-phosphate or fatty acid content in growing rats

The amount of triacylglycerol (TAG) that accumulates in adipose tissue depends on 2 opposing processes: lipogenesis and lipolysis. We have previously shown that the weight and lipid content of epididymal (EPI) adipose tissue increases in growing rats fed a low-protein, high-carbohydrate (LPHC) diet for 15 days. The aim of this work was to study the pathways involved in lipogenesis and lipolysis, which ultimately regulate lipid accumulation in the tissue. De novo fatty acid synthesis was evaluated in vivo and was similar for rats fed an LPHC diet or a control diet; however, the LPHC-fed rats had decreased lipoprotein lipase activity in the EPI adipose tissue, which suggests that there was a decreased uptake of fatty acids from the circulating lipoproteins. The LPHC diet did not affect synthesis of glycerol-3-phosphate (G3P) via glycolysis or glyceroneogenesis. Glycerokinase activity - i.e., the phosphorylation of glycerol from the hydrolysis of endogenous TAG to form G3P - was also not affected in LPHC-fed rats. In contrast, adipocytes from LPHC animals had a reduced lipolytic response when stimulated by norepinephrine, even though the basal adipocyte lipolytic rate was similar for both of the groups. Thus, the results suggest that the reduction of lipolytic activity stimulated by norepinephrine seems essential for the TAG increase observed in the EPI adipose tissue of LPHC animals, probably by impairment of the process of activation of lipolysis by norepinephrine.     

Hydrolysis of chylomicron polyenoic fatty acid esters with lipoprotein lipase and hepatic lipase

The lipolysis of rat chylomicron polyenoic fatty acid esters with bovine milk lipoprotein lipase and human hepatic lipase was examined in vitro. Chylomicrons obtained after feeding fish oil or soy bean oil emulsions were used as substrates. The lipolysis was followed by gas chromatography or by using chylomicrons containing radioactive fatty acids. Lipoprotein lipase hydrolyzed eicosapentaenoic (20:5) and arachidonic acid (20:4) esters at a slower rate than the C14-C18 acid esters. More 20:5 and 20:4 thus accumulated in remaining tri- and diacylglycerols. Eicosatrienoic, docosatrienoic and docosahexanoic acids exhibited an intermediate lipolysis pattern. When added together with lipoprotein lipase, hepatic lipase increased the rate of lipolysis of 20:5 and 20:4 esters of both tri- and diacylglycerols. Addition of NaCl (final concentration 1 M) during the course of lipolysis inhibited lipoprotein lipase as well as the enhancing effect of hepatic lipase on triacylglycerol lipolysis. Hepatic lipase however, hydrolyzed diacylglycerol that had already been formed. Chylomicron 20:4 and 20:5 esters thus exhibit a relative resistance to lipoprotein lipase. It is suggested that the tri- and diacylglycerol species containing these fatty acids may accumulate at the surface of the remnant particles and act as substrate for hepatic lipase during a concerted action of this enzyme and lipoprotein lipase.     

Adipose triglyceride lipase and hormone-sensitive lipase are the major enzymes in adipose tissue triacylglycerol catabolism

The mobilization of free fatty acids from adipose triacylglycerol (TG) stores requires the activities of triacylglycerol lipases. In this study, we demonstrate that adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) are the major enzymes contributing to TG breakdown in in vitro assays and in organ cultures of murine white adipose tissue (WAT). To differentiate between ATGL- and HSL-specific activities in cytosolic preparations of WAT and to determine the relative contribution of these TG hydrolases to the lipolytic catabolism of fat, mutant mouse models lacking ATGL or HSL and a mono-specific, small molecule inhibitor for HSL (76-0079) were used. We show that 76-0079 had no effect on TG catabolism in HSL-deficient WAT but, in contrast, essentially abolished free fatty acid mobilization in ATGL-deficient fat. CGI-58, a recently identified coactivator of ATGL, stimulates TG hydrolase activity in wild-type and HSL-deficient WAT but not in ATGL-deficient WAT, suggesting that ATGL is the sole target for CGI-58-mediated activation of adipose lipolysis. Together, ATGL and HSL are responsible for more than 95% of the TG hydrolase activity present in murine WAT. Additional known or unknown lipases appear to play only a quantitatively minor role in fat cell lipolysis.     

Millennium fat-cell lipolysis reveals unsuspected novel tracks.

Adipose tissue lipolysis, i.e., the catabolic process leading to the breakdown of triglycerides into fatty acids and glycerol, is often considered as a simple and well-understood metabolic pathway. However, progress on the hormonal regulation and molecular mechanism of fat-cell lipolysis is opening new avenues and points to a number of unanswered questions. Recent studies on the lipolytic beta- and antilipolytic alpha2-adrenergic control of lipolysis has allowed a better understanding of the relative contribution of the two types of receptors and provide strong evidence for the in vivo implication of alpha2-adrenoceptors in the physiological control of subcutaneous adipose-tissue lipolysis. A novel lipolytic system has been characterized in human fat cells. Natriuretic peptides stimulate lipolysis through a cGMP-dependent pathway. The molecular details of the lipolytic reaction are not fully understood. Translocation of hormone-sensitive lipase, the rate-limiting enzyme of lipolysis, to the lipid droplet seems to be an important step during lipolytic activation. Reorganization of the lipid droplet coating by perilipins may also facilitate the access of the enzyme. Unexpectedly, hormone-sensitive lipase-deficient mice are not obese and show residual adipose-tissue lipolysis, which suggests the existence of another triglyceride lipase. Whether the expression of this uncharacterized neutral lipase is compensatory for the lack of hormone-sensitive lipase is an important question yet to be resolved. In humans, alterations of hormone-sensitive lipase expression are associated with changes in lipolysis in various physiological and pathological states. Genetic studies show that beta2-adrenoceptor and hormone-sensitive lipase genes may participate in the polygenic background of obesity.     

Fat accumulation in the rat during early pregnancy is modulated by enhanced insulin responsiveness

Insulin sensitivity has been implicated in the variation of fat accumulation in early gestation by as-yet-unknown mechanisms. In the present study, we analyzed the insulin sensitivity of lipolysis and lipogenesis in lumbar adipocytes from rats at 0, 7, 14, and 20 days of gestation. In adipocytes of 7-day pregnant rats, we found a twofold decrease in both beta-agonist (isoproterenol and BRL-37344)-stimulated lipolysis and beta3-adrenoceptor protein but not in lipolysis initiated by forskolin or isobutylmethylxanthine, suggesting a modification of the lipolytic pathway at the receptor level. Whereas adipocytes from 7-day pregnant rats showed a twofold increase in fatty acid synthesis from glucose, those from 20-day pregnant animals displayed a decreased lipogenic activity. Insulin responsiveness of the lipolytic and lipogenic pathways was analyzed by dose-response experiments, giving evidence for the involvement of improved insulin responsiveness in the enhanced lipogenic and reduced lipolytic activities of adipocytes in early pregnancy. In contrast, insulin resistance is responsible for lower antilipolytic and lipogenic actions of insulin in late pregnant animals. In conclusion, the present study shows that enhanced adipose tissue insulin responsiveness during early pregnancy contributes to maternal fat accumulation, whereas decreased insulin responsiveness during late gestation modulates fat breakdown.     

Palmitic acid follows a different metabolic pathway than oleic acid in human skeletal muscle cells; lower lipolysis rate despite an increased level of adipose triglyceride lipase

Development of insulin resistance is positively associated with dietary saturated fatty acids and negatively associated with monounsaturated fatty acids. To clarify aspects of this difference we have compared the metabolism of oleic (OA, monounsaturated) and palmitic acids (PA, saturated) in human myotubes. Human myotubes were treated with 100μM OA or PA and the metabolism of [(14)C]-labeled fatty acid was studied. We observed that PA had a lower lipolysis rate than OA, despite a more than two-fold higher protein level of adipose triglyceride lipase after 24h incubation with PA. PA was less incorporated into triacylglycerol and more incorporated into phospholipids after 24h. Supporting this, incubation with compounds modifying lipolysis and reesterification pathways suggested a less influenced PA than OA metabolism. In addition, PA showed a lower accumulation than OA, though PA was oxidized to a relatively higher extent than OA. Gene set enrichment analysis revealed that 24h of PA treatment upregulated lipogenesis and fatty acid β-oxidation and downregulated oxidative phosphorylation compared to OA. The differences in lipid accumulation and lipolysis between OA and PA were eliminated in combination with eicosapentaenoic acid (polyunsaturated fatty acid). In conclusion, this study reveals that the two most abundant fatty acids in our diet are partitioned toward different metabolic pathways in muscle cells, and this may be relevant to understand the link between dietary fat and skeletal muscle insulin resistance.     

Genesis of fatty liver and hyperlipemia in the fetal guinea pig.

10 to 20% of [1-14C] palmitate injected into pregnant guinea pigs was recovered in lipids of their fetuses. From these data and the rate of transport of palmitate in maternal blood, it appears that placental transport of free fatty acids can account for the accumulation of lipids in late gestational fetuses. About 80% of the labeled palmitate in the fetus appeared initially in lipids of the liver. 14C appeared in plasma triglyceride fatty acids after a few minutes and subsequently accumulated in lipids of white and brown adipose tissue, suggesting that much of the palmitate deposited in adipose tissue were derived from hepatogenous triglyceride fatty acids. By contrast, 14C was usually maximal in heart and carcass lipids before it appeared in plasma triglyceride fatty acids. Lipoprotein lipase activity in fetal adipose tissue was low, and activity of cofactor protein of lipoprotein lipase in fetal blood plasma was much lower than that observed in other mammalian species. On the basis of these and earlier observations, it is concluded that the accumulation of triglycerides in liver and blood plasma of fetal guinea pigs during late gestation is at least partly the result of the large uptake of maternally derived free fatty acids by the fetal liver accompanied by rapid synthesis and secretion of triglyceride-rich very low density lipoproteins into the blood. However, limited uptake of triglyceride fatty acids in adipose tissue may contribute to the fatty liver and hyperlipemia.     

Changes in the phospholipid catabolism of mitochondria and microsomes during the development of rat liver.

The lipolytic activities of mitochondrial and microsomal fractions ('microsomes') isolated from foetal, suckling and adult rat liver were compared. The catabolism of endogenous phospholipids was followed by measuring the loss of phospholipids and the appearance of non-esterified fatty acids and lysophosphatides. The rate of mitochondrial phospholipid catabolism does not change significantly during development, but the rate of lipolysis of microsomal phospholipids increases 3-fold during development. Balance studies showed that, in mitochondria and microsomes of foetal, suckling and adult rat liver, fatty acid formation is greatly in excess of the fatty acids that can be accounted for by measuring phospholipid disappearance and lysophosphatide appearance. The hypothesis that this excess fatty acid formation resulted from the lipolysis of mitochondrial and microsomal triacylglycerols were tested and confirmed by preliminary experiments. Mitochondria and microsomes isolated from all developmental ages investigated had phospholipases with A1 and A2 activities. The degree of unsaturation of the fatty acids derived from the phospholipids of mitochondria did not vary significantly during development.     

Hormonal control of lipolysis from the white adipose tissue of hibernating jerboa (Jaculus orientalis)

1. Plasma glucose, glycerol, free fatty acids and total lipid content of the white adipose tissue were measured in euthermic and hibernating jerboa. 2. During hibernation, plasma glucose and glycerol were low compared to the euthermic animals, whereas there was no obvious difference in plasma free fatty acids. The white adipose tissue lipid content was strongly reduced in the hibernating state. 3. The effect of lipolytic hormones (norepinephrine and glucagon) and antilipolytic hormone (insulin) on in vitro glycerol release by adipose tissue isolated from hibernating or euthermic jerboa has been studied. 4. The white adipose tissue from hibernating jerboa presented a higher sensitivity to norepinephrine and glucagon than that of euthermic jerboa; insulin did not modify either basal glycerol release or lipolysis induced by the two lipolytic hormones at low temperatures (7 degrees C) and during the rewarming (from 7 degrees C to 37 degrees C) of the tissue slices. 5. These results suggested that white adipose tissue constitutes an important source of substrates derived from lipolysis during hibernation.     

Attenuation of adipocyte triacylglycerol hydrolase activity decreases basal fatty acid efflux

Fatty acids released from adipose triacylglycerol stores by lipolysis provide vertebrates with an important source of energy. We investigated the role of microsomal triacylglycerol hydrolase (TGH) in the mobilization of adipocyte triacylglycerols through inactivation of the TGH activity by RNA interference or chemical inhibition. Attenuation of TGH activity resulted in decreased basal but not isoproterenol-stimulated efflux of fatty acids from 3T3-L1 adipocytes. Lack of TGH activity was accompanied by accumulation of cellular triacylglycerols and cholesteryl esters without any changes in the expression of enzymes catalyzing triacylglycerol synthesis (diacylglycerol acyltransferases 1 and 2) or degradation (adipose triglyceride lipase and hormone-sensitive lipase). Inhibition of TGH-mediated lipolysis also did not affect insulin-stimulated Glut4 translocation from intracellular compartments to the plasma membrane or glucose uptake into adipocytes. These data suggest that TGH plays a role in adipose tissue triacylglycerol metabolism and may be a suitable pharmacological target for lowering fatty acid efflux from adipose tissue without altering glucose import.     

The presence and role of hormone-sensitive lipase in heart muscle.

Hormone-sensitive lipase (HSL) catalyses the initial, rate-limiting, reaction in adipose-tissue lipolysis. Hormone-stimulated lipolytic activity has also been observed in the heart, where endogenous triacylglycerol is the major energy store. However, the identity of the intracellular lipase responsible has yet to be established. We have partially purified a neutral lipase from bovine heart muscle and compared its properties with those of HSL from bovine adipose tissue. The heart lipase has the same subunit Mr as HSL, is immunoprecipitated by antiserum raised against purified HSL and is phosphorylated by cyclic AMP-dependent protein kinase, apparently at the same site as HSL (as judged by h.p.l.c. of tryptic phosphopeptides). Phosphorylation of the heart lipase was found to result in increased enzyme activity, demonstrating the lipase's potential to respond to hormonal stimuli. The heart lipase was shown to be present in myocytes by its immunoprecipitation from homogenates of rat myocytes by anti-HSL antiserum. These findings are consistent with the conclusion that HSL is responsible for intracellular lipolysis in heart.